The Problem With GNSS – Understanding Resilient PNT

Global Navigation Satellite Systems (GNSS) offer a great way of solving a huge range of position, navigation, and timing (PNT) problems, but it has some weaknesses that need to be considered when designing a solution.

In this article we’ll talk about the problems and how these can impact the efficacy of the system while discussing some of the potential solutions to mitigate these weaknesses.

The problem with GNSS – jamming, spoofing and incorrect data

We rely on GNSS signals, such as GPS, GLONASS, and Galileo, in more ways than we may think in our daily lives. Whether they are providing positional data for your car’s satellite navigation system, legally traceable time stamps for CCTV footage, or are being used to synchronise TV or radio broadcasts to prevent interference, GNSS signals provide a vital service that we all use.

The trouble with GNSS is that the signals are extremely weak, meaning they are very susceptible to interference, and can be blocked altogether through the use of inexpensive jamming technology. It is also possible to create false GNSS signals, using GNSS simulators, spoofing a receiver into thinking it is somewhere else. We have even seen instances where incorrect data has been uploaded to satellites, causing a ripple effect that temporarily causes GPS to transmit inaccurate signals.

The impact of losing GNSS – social and economic

Losing access to GNSS signals can cause all types of problems. For example, major financial institutions rely on GNSS to time-stamp trades to a fraction of a second, regardless of where in the world they are executed from. Without access to legally traceable time, financial markets would grind to a halt.

In 2016, there was an incident where the decommissioning of an old GPS satellite caused a 13ns delay to spread across the remaining GPS constellation which, among other things, caused a major outage of DAB radio broadcasts in the UK.

The UK government recently commissioned a report titled “The economic impact on the UK of a disruption to GNSS”, which concluded that a prolonged GNSS outage could cost the UK economy in the region of £1bn per day.

The impact of losing access to GNSS could well go beyond matters of inconvenience or economics, it could even result in risk to life. Distress beacons, emergency services and communications networks all rely on GNSS, so any prolonged GNSS outage could have serious consequences beyond what you might expect.

GNSS resilience – solutions for more reliable PNT

Satellite Time & Location (STL) is a PNT system using the Iridium satellite constellation. It broadcasts an encrypted signal that is around one thousand times stronger than traditional GNSS, meaning signals can be received indoors, are very difficult to jam, and are currently impossible to spoof.

Broadshield is a collection of software algorithms that monitor a GNSS signal and can detect anomalies in that signal, such as those caused by jamming or spoofing attacks, rapidly disregarding any false signal and switching the timing server over to a different ‘known good’ reference, or reverting the internal oscillator.

eLoran is a land-based system that uses high-power, long-wave radio signals to provide PNT data to a receiver. eLoarn signals are much more difficult to jam than GNSS, making it a good augmentation to GNSS for timing applications.

Anti-jamming antennas offer a relatively simple way of preventing jamming and spoofing attacks from ground-based transmitters. They are designed to block any signals below a certain level, meaning they are essentially only able to receive GNSS signals from the sky.

Multi-constellation GNSS support reduces reliance on a single constellation, such as GPS. With support for GPS, Galileo, GLONASS, BeiDou and QZSS, our PNT systems can fall-back to an alternative constellation in case of any spoofing attacks, or if any one GNSS constellation experiences an outage.

Testing a GNSS receiver to see how it copes with jamming or spoofing attacks in a repeatable way is vital when designing new systems. Our range of GNSS simulators offer this level of functionality, while supporting all major constellations.

An internal oscillator provides the most basic backup for GNSS in timing applications, minimising drift from UTC if any interruption to GNSS does occur.

Support for external IMUs means that GNSS positional data can be complimented with inertia-based positional data, helping to minimise location inaccuracy caused by any loss of GNSS signals.

SAASM encrypted GPS support for timing and positioning systems can be used in military applications to protect against spoofing attacks, while providing higher positional accuracy than standard commercial GPS.

To learn more about out resilient PNT solutions, please contact one of the team on 01527 512400, or email us at computers@steatite.co.uk